A system of this type is disclosed in DE 38 39 364 C2. Each of the rod lenses has on its first rod end a marginal cylinder whose diameter is adjusted to the inner diameter of the endoscope tube. The marginal cylinder has a length of about two-thirds the total length of the rod lens and serves to store the rod lens in the endoscope tube. The second rod end likewise includes a marginal cylinder whose diameter, however, is smaller than the inner diameter of the endoscope tube. The sudden transition from a thicker to a thinner rod lens part increases the danger of breakage at this spot during processing and during insertion of the endoscope tube.
The end surface of the thicker marginal cylinder is convex. At the end surface of the thinner marginal cylinder, a corrective lens is cemented. The convex end surfaces point to an intermediate image plane. The corrective lenses enclose a lens aperture.
It is known in the prior art that rod lenses in the area of cemented surfaces, when the endoscope tube is subject to bending, are exposed to increased danger of breakage. Reducing the rod diameter in this area is intended to avoid any impact from the bending in the cemented area. For configuring the aperture area, however, it is necessary that a distance holder should be inserted between the two rod lenses. This distance holder takes the form of a small tube, which on the one hand is contiguous to the endoscope tube and on the other hand is supported on the corrective lenses in the axial direction. During the exertion of bending pressure of the endoscope tube, it therefore is unavoidable that radially varying axial pressures are exerted on the cemented corrective lenses by the small distancing tube. These pressures can lead to damaging of the cemented surface or of the end surface and thus to a decline in the optical imaging quality.
Bending pressure on the endoscope tube also acts directly on the relatively long, thicker marginal cylinder and can result in breaking of the rod lens.
It is therefore the object of the invention to configure the rod lenses in such a way that their fragility when positioned in an endoscope tube is minimized and to provide a distance holder adapted thereto.